Water-marking methods are likewise known by the term “water-marking”. The objective of these water-marking methods is to encode an identifier in the digital book in a manner that is not perceptible to users. This identifier is then used for, by way of example, identifying the user who has provided a copy of a digital book on multimedia content sharing networks. Thus, the water-marking of a digital book with an identifier from the purchaser may dissuade this purchaser from distributing the digital book that he has purchased for free.
A digital book contains a pre-existing text in which each character is coded by at least one code and a pre-existing typeface or a reference to a pre-existing typeface. Each typeface has code/glyph pairs. Each code/glyph pair associates a code from the coded text with a glyph that is used for displaying a character on a screen.
In microcomputing, since all styles and all bodies can be produced on the basis of the vectorial representation of a typeface, the term “typeface” denotes a type of character line, without taking account of the style, the boldness and the body. Type is understood to mean the form of the character line as identified by a name such as “Times”, “Courier”, “Arial”, . . . etc.
“Style” denotes the degree of inclination of the characters such as inclinations known by the terms “roman” or “italic”.
“Boldness” denotes the thickness of the line such as thicknesses known by the terms “light”, “semi-bold”, “bold”.
“Body” denotes the size of the characters expressed in points such as 10, 12, 14, . . . etc.
A font is a set of glyphs, that is to say visual representations of characters, from one and the same typeface, having the same style, body and boldness. Thus, generally, a typeface comprises multiple fonts.
Usually, a typeface associates a code with each glyph from a character. The codes are generally defined by a standard such as the Unicode standard.
Equally, there are today several electronic formats of typefaces such as the TrueType and OpenType formats. These TrueType and OpenType formats are vector typeface formats.
A vector typeface is a typeface in which each glyph is defined not by a dot matrix image but rather by equations for one or more curves. The curves are typically Bézier curves. With vector typefaces, it is possible to increase the size of the character without any step effect appearing when they are displayed.
Known water-marking methods for an identifier involve:                development of a new text on the basis of the pre-existing text, by replacing, in the pre-existing text, a first code from the pre-existing typeface with a second, different code from the pre-existing typeface, this second code being associated with a glyph that is very similar or identical to that associated with the first code, then        provision of the new text and of the pre-existing typeface as a water-marked digital book.        
The first codes from the pre-existing text that are replaced by the second codes are selected as a function of the value of the identifier to be encoded. Thus, the new text has both first and second codes for displaying the same glyph on the screen or very similar glyphs. It is the position of the first and second codes, relative to one another, in the new text that encodes the value of the identifier in the digital book water-marked by this method.
Such a water-marking method is described in the application WO 2011/021 114, for example.
However, the known methods are not robust towards collusive attacks. Indeed, by comparing the coded text contained in various copies of the water-marked digital book with different identifiers, it is fairly simple to pinpoint the codes from the coded text that have been modified. This information is then used to reconstruct the original coded text, which does not have the water-marking. By way of example, this information makes it possible to replace the second codes with the first codes in order to reconstruct the original un-water-marked text.
The prior art likewise discloses:                WO02/103461 A2,        US2004/001606.        